We are using mouse embryonal carcinoma (EC) cell cultures to study the genetic and epigenetic controls on the transition from embryonic stem cells to differentiated cells. The H6 EC line can be induced to aggregate and then to compact by regulating the concentration of calcium in the growth medium, and induced to differentiate by exposure to retinoic acid. These three processes are being examined since they have features which parallel early developmental stages of the preimplantation mouse embryo and therefore could lead to a better understanding of normal and abnormal development. Stem cell aggregation is being studied by utilizing genetic variants in which the inability to aggregate is associated with altered membrane glycoprotein patterns. Compaction of the H6 aggregate is being characterized functionally and ultrastructually. The controls over compaction and its significance to subsequent cell differentiation are being pursued. The retinoic-induced differentiation of H6 stem cells to endoderm-like cells is being studied by achieving rescue from the terminal state through cell hybridization or DNA transfection. Recent data suggest the existence of specific gene(s) able to return terminally differentiating cells to a stem cell state.